The field of this invention relates to oxidation catalysts for the production of maleic anhydride as well as processes for the manufacture of phosphorus-vanadium, and phosphorus-vanadium-co-metal catalysts suitable for the oxidation of butane to maleic anhydride.
Maleic anhydride is of significant commercial interest throughout the world and is extensively used in the manufacture of alkyd resins. It is also a versatile intermediate for chemical synthesis. Consequently, large quantities of maleic anhydride are produced each year to satisfy these needs. The production of maleic anhydride by the catalytic oxidation of benzene and butene is well known, and until recently the principal method employed for the manufacture of maleic anhydride was by the air oxidation of benzene in the presence of certain heavy metal oxide catalysts. However, because of the inherent toxicity of benzene fumes, the trend has been to eliminate the utilization of benzene as a feedstock and newer facilities tend to utilize butane oxidation processes.
The present invention provides an oxidation catalyst for the manufacture of maleic anhydride. Further, the present invention provides a process for the manufacture of a phosphorus-vanadium, and phosphorus-vanadium-co-metal oxide catalysts by carrying out the reaction in a glycol ether solvent using phosphoric acid as a source of phosphorus.
The present invention provides a process for the manufacture of maleic anhydride in the presence of the catalyst manufactured by the novel process.
Vanadium phosphate butane oxidation catalysts are known and are described in U.S. Pat. Nos. 4,515,904, 4,652,543, and 4,732,885, all of which are incorporated herein by reference. The catalyst of the present invention provides several economic advantages over the prior vanadium phosphate butane oxidation catalysts. The prior catalysts use halogen-containing compounds, for example, chlorine-containing compounds. The inventive catalyst of the present invention does not use chlorine-containing compounds and, therefore, does not require expensive chlorine corrosion resistant manufacturing equipment. Further, chlorinated hydrocarbon byproducts are typically formed by the prior catalyst processes when the chlorine-containing compounds attack the solvent system used in the preparation of the catalyst. Since these chlorinated hydrocarbon byproducts are not formed by the present catalyst process, the solvent can be recycled and used to prepare additional catalyst. Thus, solvent supply costs and solvent disposal costs are greatly reduced.
In addition, while a calcination step may add improved properties to the catalyst of the present invention, the calcination step is not necessary for the practice of the present invention. Thus, one step of the prior catalyst preparation process can be eliminated to improve both efficiency and cost.
Briefly, our catalyst is suitably prepared in glycol ether solvents by:
(1) slurrying vanadium, phosphoric acid, and any additional metals or metal oxides with a glycol ether solvent; PA1 (2) refluxing the slurry at from about 0.degree. C. to about 200.degree. C. for about 0.5 hours to about 6 hours, to reduce the vanadium from a plus five oxidation state to a plus four oxidation state which typically can be detected by a color change; PA1 (3) drying the catalyst to substantially remove the solvent which can include standard drying techniques well known to those skilled in the art, for example oven or rotary drying; PA1 (4) shaping the catalyst in the desired geometric shape; and PA1 (5) activating the catalyst.
The vanadium compound can be vanadium pentoxide, vanadium tetrachloride, vanadium trichloride, vanadium oxydichloride, vanadium oxytrichloride, vanadium tetraoxide, vanadium oxalate, and most soluble vanadium complexes. Additionally, suitable vanadium compounds include: vanadium oxides, such as vanadium trioxide and the like; vanadium oxyhalides, such as vanadyl chloride, vanadyl dichloride, vanadyl trichloride, vanadyl bromide, vanadyl dibromide, vanadyl tribromide and the like; vanadium-containing acids, such as meta-vanadic acid, pyrovanadic acid and the like; vanadium salts, such as ammonium meta-vanadate, vanadium sulfate, vanadium phosphate, vanadyl formate, vanadyl oxalate and the like; however, vanadium pentoxide is preferred.
In our catalyst preparation, various hydrous phosphoric acids (for example, 85% orthophosphoric acid) and anhydrous phosphoric acids (for example, 100% to 120% phosphoric acid) may be used. Suitable phosphoric acids including orthophosphoric acid, pyrophosphoric acid, triphosphoric acid, metaphosphoric acid, phosphorus pentoxide, and the like, or blends thereof. Orthophosphoric acid and polyphosphoric acids have been found to give a particularly useful catalyst. Polyphosphoric acids are phosphoric acids that are typically recognized to be a mixture of phosphoric acids, for example, a polyphosphoric acid that is commercially available from Albright and Wilson, Richmond Va., is approximately 54% H.sub.3 PO.sub.4, 41% pyrophosphoric acid, and 5% triphosphoric acid.
The slurry can include additional metals or metal oxides such as molybdenum oxide, zinc oxide, uranium oxide, tungsten oxide, tin oxide, bismuth oxide, titanium oxide, chromium oxide, zirconium oxide, niobium oxide, antimony oxides and cobalt oxide in organic solvents, preferably organic ether solvents.
Suitable glycol ether solvents can be represented by Formula I EQU R.sub.1 O(R.sub.2 O).sub.n H I
wherein R.sub.1 is a C.sub.1 to C.sub.12 alkyl or aryl moiety; R.sub.2 is a C.sub.2 to C.sub.12 alkyl or aryl moiety; and n is from 1 to 5.
Preferably, C.sub.3 to C.sub.8 glycol ethers are used in the present invention. Most preferably, C.sub.3 to C.sub.6 glycol ethers are used in the present invention. Higher carbon number glycol ethers may result in logistical problems in the manufacture process. However, these problems may be avoided by further optimization of the reaction conditions. The solvents of Formula I are preferably liquids at ambient temperature.
Suitable solvents, according to Formula I, include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, diethylene glycol monomethyl ether, propylene glycol monomethyl ether, and the like. We have found that ethylene glycol monomethyl ether (methoxyethanol), propylene glycol monomethyl ether (methoxypropanol), and diethylene glycol monomethyl ether are cost effective when used in the present invention.
Further, a cosolvent system can be prepared where two or more solvents of Formula I are present. For example, an ethylene glycol monomethyl ether and propylene glycol monomethyl ether cosolvent system can be used as shown in Example 8.
While the reaction solution is being refluxed, if desired a modifier or mixture of modifiers such as xylene, m-xylene, p-xylene, benzene, toluene, mesitylene, pseudocumene, phthalic anhydride, trimellitic anhydride, benzoic acid, toluic acid, phthalic acid isophthalic acid, terephthalic acid, trimesic acid, or trimellitic acid, is suitably added to the reaction solvent. After refluxing, the color of the syrup is typically light blue. The volume of the solution is dried, typically at a temperature of from about 120.degree. C. to about 150.degree. C. and from about 0 to about 30 inches of mercury vacuum under an optional air purge. Once dry, the color of the solid material is typically from blue-green to dark brown. The catalyst can be formed into geometric forms, such as cylinders, using graphite, Sterotex, or other lubricants such as steric acid, zinc stearate, or starch and binders such as polyvinyl alcohol.
The two important requirements of a good catalyst are low pressure drop and high yield. The size and shape of the tablets determine the void fraction available in a particular reactor. The void fraction should be large enough to avoid development of a large pressure drop across the reactor. One suitable catalyst form is a cylinder whose length and diameter are roughly equivalent and range in size from about 1/16 inch to about 1/2 inch
In addition to its dependence on the shape and tablet dimensions, the reactors void fraction depends on whether these dimensions change at the high temperatures required for efficient conversion of butane. For example if the tablet undergoes a volume increase or "expansion," the void fraction will decrease and an unacceptable increase in pressure drop will result. Processes for reducing tablet expansion are known, for example, see U.S. Pat. Nos. 4,933,312, 4,957,894 and 5,019,545, all of which are fully incorporated herein by reference.
The catalyst in the form of geometric shapes or in powder form can be calcined in air or a nitrogen-air combination before loading into a suitable reactor, typically a tubular reactor. Calcining can be in dry or humid air, preferably in humid air. For example, see Example 16.
We have found that the introduction of water (from about 2 to about 100 wt. % of the total mass) into air during calcination results in faster catalyst activation and a higher maleic anhydride yield.
The catalyst is activated further by the addition of water and phosphorus compounds or mixtures thereof, such as alkylphosphates, phosphates, and phosphines, activating the catalyst by the addition of butane or another hydrocarbon feedstock and a phosphorus compound at a temperature of about 300.degree. C. to about 500.degree. C. Representative phosphorus compounds have the following structure: ##STR1## wherein R is a phenyl or an alkyl radical of 1 to 6 carbon atoms and X is H or R. Suitable compounds are primary, RPH.sub.2, secondary, R.sub.2 PH, and tertiary, R.sub.3 P, phosphines, such as ethyl phosphine; the tertiary phosphine oxides, R.sub.3 PO, such as tripropyl phosphine oxide: the primary, RP(O)(OX).sub.2, and secondary, RP(O)OX, phosphonic acids, such as benzene phosphonic acid; the esters of the phosphonic acids, such as diethyl methanephosphonate; the phosphonous acids, RPO.sub.2 X.sub.2, such as benzenephosphonous acid and the esters thereof, such as the monoethyl ester; the primary, ROP(OX).sub.2, secondary, (RO).sub.2 POX, and tertiary, (RO).sub.3 P, phosphites, such as diethyl phosphite, trimethyl phosphite, triethyl phosphite, triisopropyl phosphite, tripropyl phosphite, and tributyl phosphite, and the pyrophosphites, such as tetraethyl pyrophosphite. The preferred phosphous compound is an ester of orthophosphoric acid having the formula (RO).sub.3 P=O wherein R is hydrogen or a C.sub.1 to C.sub.4 alkyl, at least one R being a C.sub.1 to C.sub.4 alkyl. The preferred phosphorus compounds are triethylphosphate and trimethylphosphate.
The novel catalyst further comprises a phosphorus-vanadium mixed oxide or a phosphorus-vanadium mixed oxide promoted by metals. The atomic ratio of the vanadium to phosphorus can suitably be in the range of 0.5:1 to 1.25:1.0. The total atomic ratio of vanadium to phosphorus advantageously is in the range of 0.75:12 to 1:1. It is preferred that the total atomic ratio of molybdenum, zinc, tungsten, uranium, tin, bismuth, titanium, niobium, or cobalt to vanadium should be in the range of 0.001:1 to 0.2:1. The atomic ratio of phosphorus to vanadium is suitably in the range of 0.8:1 to 2:1, preferably 1:1 to 1.3:1.
The co-metal, such as molybdenum, zinc, tungsten, uranium, bismuth, titanium, zirconium, antimony, niobium, cobalt, chromium, or tin may be added as a compound together with vanadium, or separately introduced into the solution. Suitable co-metal compounds comprise their oxides and soluble salt. Suitable molybdenum compounds comprise molybdenum oxide and most soluble molybdenum salts. If it is desired to improve physical properties of the catalysts, they may be treated with the suspension of an inert support, for example, alumina, titania, silicon carbide, kieselguhr, pumice, or silica. The catalyst may be reinforced with such materials at any stage in its preparation.
The oxidation of butane to maleic anhydride may be accomplished by contacting n-butane in low concentration in oxygen with the described catalyst. Air is entirely satisfactory as a source of oxygen, but synthetic manufacturers of oxygen and diluent gases, such as nitrogen also may be employed. Air enriched with oxygen may be used.
The gaseous feed stream to the oxidation reactors will normally contain air and about 0.2 to about 1.7 mole percent of n-butane. About 0.8 to 1.5 mole percent of n-butane is satisfactory for optimum yield of maleic anhydride for the process of this invention. Although higher concentrations may be employed, explosive hazards may be encountered. Lower concentrations of butane, less than about one percent, of course, will reduce the total yield obtained at equivalent flow rates and, thus, are not normally economically employed. The flow rate of the gaseous stream through the reactor may be varied within rather wide limits, but preferred range of operations is at the rate of from about 100 cc to about 4000 cc of feed per cc of catalyst per hour, and more preferably from about 1000 cc to about 2400 cc of feed per cc of catalyst per hour. Residence times of the gas stream will normally be less than about four seconds, more preferably less than about one second, and down to a rate where less efficient operations are obtained. The flow rates and residence times are calculated at standard conditions of 760 mm of mercury at 25.degree. C.
A variety of reactors will be found to be useful, and multiple tube heat exchanger-type reactors are quite satisfactory. These reactors may have a single or multiple cooling zones, for example a dual zone system. The diameter of such reactor may vary from about one-quarter inch to about three inches, and the length may be varied from about three to about sixteen or more feet. The oxidation reaction is an exothermic reactor and, therefore, relatively close control of the reaction temperatures should be maintained. It is desirable to have the surface of the reactors at relatively constant temperatures, and some medium to conduct heat from the reactors is necessary to aid temperature control. Such media may be Woods metal, molten sulphur, mercury, molten lead and the like, but it has been found that eutectic salt baths are completely satisfactory. One such salt bath is a sodium nitrate, sodium nitrite, potassium nitrate eutectic constant temperature mixture. An additional method of temperature control is to use a metal block reactor whereby the metal surrounding the tube acts as a temperature regulating body. As will be recognized by one skilled in the art, the heat exchanger medium may be kept at the proper temperature by heat exchangers and the like. The reactor or reaction tubes may be iron, stainless steel, carbon steel, nickel, glass tubes, such as vycor and the like. Both carbon steel and nickel tubes have excellent long life under the conditions of the reaction described herein. Normally, the reactors contain a preheat zone under an inert material such as one-quarter inch alundum pellets, inert ceramic balls, nickel balls, or chips and the like present at about one-half to one-tenth the volume of the active catalyst present.
The temperature of reaction may be varied within some limits, but normally the reaction should be conducted at a temperature within a rather critical range. The oxidation reaction is exothermic and once reaction is underway, the main purpose of the salt bath or other media is to conduct heat away from the walls of the reactor and control the reaction. Better operations are normally obtained when the reaction temperature employed is no greater than from about 20.degree. F. to about 50.degree. F. above the salt bath temperature. The temperature of the reactor, of course, will also depend to some extent upon the size of the reactor and the butane concentration.
The reaction may be conducted at atmospheric, super-atmospheric, or below atmospheric pressure. The exit pressure will be at least slightly higher than the ambient pressure to ensure a positive flow from the reactor. The pressure of the inert gases must be sufficiently high to overcome the pressure drop through the reactor.
Maleic anhydride may be recovered by a number of ways well known to those skilled in the art. For example, the recovery may be by direct condensation or by absorption in suitable media, with specific operations and purification of the maleic anhydride.